The invention relates to gearsets for motor vehicle transmissions, particular to Ravigneaux gearsets for automatic transmissions.
A conventional Ravigneaux gearset includes a single carrier containing two sets of planet pinions, long pinions, and short pinions. Each pinion set includes three pinions, each pinion being mutually spaced angularly about a central axis, each short pinion meshing with a corresponding long pinion. There are two sun gears, a short sun gear meshing with the three short pinions, and a long sun gear meshing with the three long pinions. There is one ring gear, which meshes with the three long pinions.
Both planet pinion sets are supported for rotation on the carrier. Each long pinion is in mesh with a short pinion, the ring gear, and the long sun gear. Each short pinion is in mesh with a long pinion and the short sun gear.
Ravigneaux gearsets are used in many automatic transmissions due to their compact size. A Ravigneaux gearset can be used in lieu of two simple planetary gearsets to produce four forward speeds and a reverse gear. They are smaller, and require fewer components than two simple planetary gearsets because they have a common pinion, one ring gear, and one carrier. Their manufacturing and assembly costs compare favorably to those of simple planetary gearsets.
Ravigneaux gearsets, however, are limited in torque capacity due to the pairing requirement for the pinions. Unlike conventional planetary gear sets, which can be made more robust by adding an extra pinion, the pairing requirement of a Ravigneaux gearset requires that two pinions be added to increase torque capacity. Because there is not enough space in the annular zone surrounding the carrier to add a long pinion and a short pinion, the toque and power capacity of a Ravigneaux gearset is usually increased by making the gearset larger, widening the gears and pinions and increasing the diameter of the shafts, or by incorporating premium processes to the manufacture of the gears, pinions and carrier. But this increases the cost weight and package size of the transmission. Heretofore, Ravigneaux gearsets have been limited to three pairs of planet pinions, a total of six pinions.
There is a need to increases the power and torque capacity of a Ravigneaux gearset, preferably by adding pinions, but without increasing the size of the gearset.